1. Field of the Invention
The present invention relates to an electrooptical device, such as a liquid-crystal display device or an EL (electroluminescence) display device, and a flexible wiring board appropriate for use in these electrooptical devices. The present invention also relates to electronic equipment incorporating the electrooptical device.
2. Description of the Related Art
As the demand for more compact and thinner design is currently mounting in electronic equipment, high-density wiring and thin design is required of a flexible printed circuit (FPC) board used in the electronic equipment.
A variety of manufacturing methods for the flexible printed circuit board are known. Known as a manufacturing method for the flexible printed circuit board based on a double-sided copper-foil laminated flexible board is a subtractive method, for instance. The manufacturing method for the flexible printed circuit board based on the subtractive method is now discussed.
(1) First, a hole, i.e., a through-hole is drilled in the double-sided copper-foil laminated flexible circuit board using a drill. (2) In succession, the surface of the copper foil and the hole are plated through electroless plating. (3) After a photoresist or a resist ink is applied over the plated surface, a resist layer is produced through patterning to leave an unnecessary portion as a circuit. (4) In succession, an exposed area expected to become a circuit is thickened with a conductor such as copper or the like through electroplating. (5) After the conductor is plated with gold or solder, the resist layer is peeled off. In this state, the area where the conductor is formed, and the through-hole are covered with gold or solder. (6) Etching is then performed by using an etchant which dissolves neither gold nor solder but copper only, and the circuit is thus produced. (7) An overcoat ink or a cover film is formed, as required.
The following problems arise when the flexible printed circuit board is fabricated from the double-sided copperfoil laminated flexible circuit board.
In the manufacturing method, through-holes need to be created to electrically connect circuits formed on the double sides of the double-sided copper-foil laminated flexible circuit board. Since the through-holes are drilled by a drill through a mechanical drilling step, the efficiency of manufacturing process is low. Since electrical connection is performed using a plating technique subsequent to the drilling operation, a number of manufacturing steps, such as the cleaning of the inner wall of the hole, the pre-process of the electroless plating, electroless plating, and electroplating for thickening the conductor, are needed involving a great deal of time, productivity is lowered, and a costly wiring board thus results.
Since the mechanical drilling step using the drill sets a limit to the effort to promote the miniaturization of a contact section in size, a high-density wiring pattern is not accomplished.
It is an object of the present invention to provide a flexible wiring board which is of high productivity and low cost, while permitting a wiring pattern to be miniaturized. It is another object of the present invention to provide an electrooptical device and electronic equipment, each incorporating the flexible wiring board.
A flexible wiring board of the present invention includes a first single-sided flexible board and a second single-sided flexible board,
wherein the first single-sided flexible board comprises a first base body having an insulative property, and a first wiring layer formed in a predetermined pattern on the first base body,
the second single-sided flexible board comprises a second base body having an insulative property, and a second wiring layer formed in a predetermined pattern on the second base body,
an insulating layer for covering the wiring layer is formed on at least one of the first and second single-sided flexible boards, a hole, forming a contact section, is created in the insulating layer to electrically connect the first wiring layer and the second wiring layer within a predetermined area, and
the first single-sided flexible board and the second single-sided flexible board are arranged with the first wiring layer and the second wiring layer in a facing state, and are bonded through an anisotropically conductive adhesive layer.
In this flexible wiring board, the contact section for electrically connecting the two wiring layers includes the hole formed in the insulating layer. Unlike the double-sided copper-foil laminated flexible board, this arrangement eliminates the need for the formation of the through-hole, and allows the hole to be formed through a photolithographic technique. In the flexible wiring board of the present invention, a fine miniature contact section is formed, achieving a miniaturization of the wiring pattern. Compared to the through-hole with the inside thereof plated to form a conductor layer, the flexible wiring board of the present invention needs neither large number of steps nor great deal of time, and productivity is increased, and low-cost design is thus implemented.
The second single-sided flexible board is preferably arranged on a portion of the first single-sided flexible board.
This arrangement provides the following operation and advantages.
(1) The second single-sided flexible board is arranged on only an area where it is needed from the wiring design point of view. This arrangement achieves a reduced cost of the entire board, compared to the double-sided board that requires a full-size flexible wiring board.
(2) Flexibility of the flexible wiring board is controlled in the manufacture of the second single-sided flexible board, in addition to the design of wiring thereof. Specifically, in addition to considering the design of wiring, the strength and flexibility (so-called stiffness) of the flexible wiring board are selectively controlled by setting the formation area of the second single-sided flexible board.
(3) With the second single-sided flexible board partly arranged, the area where no second single-sided flexible board is formed has the thickness of the first single-sided flexible board only. By setting the formation area of the second single-sided flexible board, the thickness of the flexible wiring board in a bent portion thereof is set to be equal to the thickness of the single-sided flexible board. Compared to a double-sided board, the flexible wiring board having such a construction reduces space required for the bent portion of the flexible wiring board, thereby contributing to a thin design of electrooptical devices.
The first single-sided flexible board and the second single-sided flexible board are implemented in the following modes.
(A) The first single-sided flexible board may have a shape matching the general configuration of the flexible wiring board. In such an embodiment, a general wiring of the flexible wiring board is fabricated in the first single-sided flexible board. For instance, an input terminal region and an output terminal region may be arranged in the first single-sided flexible board.
(B) A portion of the second single-sided flexible board may be bonded onto the first single-sided flexible board while the remaining portion thereof may remain unbonded to the first single-sided flexible board. In this embodiment, the wiring design of the second single-sided flexible board is diversified. For instance, the first single-sided flexible board may include an input terminal region and a first output terminal region, and the second single-sided flexible board may include a second output terminal region.
(C) The first single-sided flexible board and the second single-sided flexible board may respectively include the insulating layers. This embodiment assures electrical insulation between the first wiring layer and the second wiring layer. As long as electrical insulation is assured, arranging the insulating layer on one of the first and second single-sided flexible boards is acceptable.
The anisotropically conductive adhesive layer may be formed by thermocompression bonding the first single-sided flexible board and the second single-sided flexible board with an anisotropically conductive film interposed therebetween.
An electrooptical device of the present invention includes an electrooptical material layer between mutually opposing first and second substrates,
wherein the first substrate comprises a first wiring bonding region which does not overlap the second substrate,
the second substrate comprises a second wiring bonding region which does not overlap the first substrate, and
at least one of the first wiring bonding region and the second wiring bonding region is connected to the flexible wiring board according to the present invention.
The electrooptical material layer may be a liquid-crystal layer.
Electronic equipment of the present invention may include the electrooptical device of the present invention.
The electrooptical device and the electronic equipment of the present invention include the flexible wiring board of this invention and implement low-cost and thin-structure design, taking advantages of the operation and advantage of the flexible wiring board.